1. Field of the Invention
The present invention relates to constant velocity universal joints for use in drive line applications, that is to say particularly for automotive use, e.g. in the drive shafts of front wheel drive vehicles, but also in any applications in which it is desired to transmit rotational movement between two rotary shafts which are inclined to one another, at least at certain times.
2. Description of the Prior Art
The best known universal joint is probably the Cardan joint which comprises two yokes which are at right angles to one another and located at the ends of respective shafts and in whose ends are respective bores which extend perpendicular to the axis of the respective shaft and in which the ends of two opposed arms of a common cross piece are rotationally located. Such a joint can transmit rotational motion through a substantial angle and whilst the speed of the two shafts is always the same expressed in revolutions per minute, the speed of the output shaft varies cyclically over each revolution as the input shaft rotates at a constant speed. The reason for this is that the Cardan joint does not satisfy the basic relationships necessary if the instantaneous velocity of the output shaft is to be constant at all times with a constant input speed, namely firstly that the plane of contact between the two shafts must be held constant in space, secondly that the plane of contact must be perpendicular to the plane determined by the rotational axes of the two shafts and thirdly that the plane of contact must define an angle with the axis of each shaft which is equal to one half of the total joint angle.
Near constancy of velocity is achieved by the Double Cardan Universal Joint which comprises two Cardan joints in series, the output member of the first joint comprising the input member of the other joint. The ends of the two shafts are provided with cooperating support and centering means. The joint operates as a true constant velocity joint at one joint angle only but at other joint angles the variation in the output velocity is very much less than with e.g. a single Cardan joint. However, even this reduced velocity variation is unacceptable for many applications.
The necessary relationships for true constancy of velocity are achieved by the Rzeppa joint which comprises inner and outer races drivably connected by a plurality of balls which are positioned in the constant velocity plane by axially offset meridionally curved grooves and are maintained in this plane by a cage located between the two races. The torque transmitted by this joint is transmitted through the very small contact areas of the balls which skid in contact with the races, when the joint is deflected, and thus, at appreciable joint angles, the joint is subjected to very high wear. In practice, the spherical surfaces of the balls are in contact with the convex internal surfaces of the races which means that the contact is essentially point contact and thus that the surface stresses are extremely high. This is tolerable in practice only because high joint angles are only rarely used in most applications and then only for short periods of time. However, if a Rzeppa joint is operated at a significant joint angle and at high speed and/or high torque for an appreciable period of time wear and the generation of heat and noise are substantial and the joint is subject to premature failure.
German Patent No.914208 discloses a constant velocity joint in which two part-cylindrical recesses are formed in the opposed end of the two shafts. Slidably received in each recess is a part-cylindrical portion of a respective member or spider of generally U shape, at the end of each of whose limbs is an aperture defined by a cylindrical portion, the axis of which is perpendicular to that of the associated recess. Rotatably received in all four apertures is a central joint pin. Slidably received on the joint pin is a guide member which is keyed to two guide pins which are in sliding engagement with the internal surface of a respective part-cylindrical portion and are rotatably secured to the end of a respective one of the shafts.
Torque is transmitted through this known joint through the engaging surfaces of the joint pin and the spiders. Due to the fact that the engaging surfaces of the spiders are cylindrical the central pin is necessarily relatively small because otherwise the overall size of the joint would become unacceptably large. This means that the engaging surfaces are necessarily of small area which results in high surface loadings which in turn results in lubrication breakdown and in galling and ultimately welding or seizure of the engaging surfaces, if the joint is transmitting significant torque. Furthermore, when the two shafts are inclined, the guide member reciprocates longitudinally on the guide pin which means that the joint is not balanced and this results in unacceptable vibration when the joint rotates at high speed.
Accordingly it is an object of the invention to provide a constant velocity joint which overcomes the problems of the known joints and is cheap and simple to manufacture and which is capable of operating at high speeds and joint angles for an extended period of time without being subject to appreciable wear or the risk of premature failure. It is a further object to provide a constant velocity joint which is at least approximately statically and dynamically balanced at all times.